CAPTURE ZONES FOR PASSIVE WELLS HETEROGENEOUS UNCONFINED AQUIFERS

Many piezometers/wells produce at such low rates and frequency that th eir impact on local hydraulics is negligible, e.g. monitoring piezomet ers or homeowner wells installed in highly permeable soils. Defining a deterministic capture zone for such wells is often of limited utility as the capture zone is effectively a single flow line directed upgrad ient. In order to provide more insight into the capture zone of such w ells, a statistically based capture zone, termed a ''percentile captur e contour'' (PCC), is introduced and analyzed. The capture zone is def ined by quantifying, for a given travel time, the variation of the len gth and orientation of the flow line emanating upgradient from the wel l. Capture zone variation herein depends on second-order stationary ra ndom hydraulic conductivity fields and is calculated using Monte Carlo analysis. Monte Carlo analysis yielded increases in mean travel dista nces as the variance and/or Integral scale in log K increased, but dec reases as the angle between the principal direction of the correlation structure and the regional flow increased. The average travel distanc e exceeded the travel distance estimated by a homogeneous solution usi ng the geometric mean hydraulic conductivity. Transverse variation dep ended both on the variance and integral scale of log K, but was insens itive to the orientation of the principal correlation direction. The m ean orientation of the flow path varied with the principal direction o f the correlation structure, deviating up to 20 degrees from the orien tation of the hydraulic gradient. These observations are consistent wi th flow following preferential pathways and indicate that significant uncertainty exists for source prediction of water feeding passive well s.